Apparatuses having at least one oscillatory element, so-called vibration detectors, are already known for detecting, or monitoring, the fill level of a fill substance in a container. The oscillatory element is usually at least one oscillatory rod, which is secured to a membrane. The membrane is excited to oscillate via an electromechanical transducer, e.g. a piezoelectric element. Because of the oscillations of the membrane, the oscillatory element secured to the membrane also oscillates.
Vibratory detectors embodied as fill level measuring devices make use of the effect wherein the frequency and amplitude of the oscillations depend on the current degree of covering of the oscillatory element—while the oscillatory element can oscillate in air freely and without damping, it experiences a frequency and amplitude change, as soon as the fill substance rises to cover it partially or completely. On the basis of a predetermined frequency change (usually, the frequency is measured), an unequivocal deduction can then be drawn concerning the reaching of a predetermined fill level of the fill substance in the container. Fill level measuring devices are primarily used as protection against overfilling or for the purpose of protecting against pumps running empty.
The damping of the oscillation of the oscillatory element is also influenced by the present density of the fill substance. Therefore, at constant degree of covering, there is a functional relationship with the density of the fill substance, so that vibration detectors are best suited both for fill level, and also for density, determination. In the practice, for the purposes of monitoring and detecting fill level, or density, of the fill substance in the container, the oscillations of the membrane are registered and converted by means of at least one piezoelement into electrical, received signals. The electrical, received signals are then evaluated by an evaluation electronics. In the case of fill level determination, the evaluation electronics monitors the oscillation frequency and/or the oscillation amplitude of the oscillatory element and signals the state ‘sensor covered’, or ‘sensor uncovered, as soon as the measured values, respectively, fall below, or exceed, a predetermined reference value. A corresponding report to the operating personnel can occur optically or acoustically. Alternatively or supplementally, a switching event is triggered; thus, perhaps, an inlet, or outlet, valve on the container is opened or closed.
DE 100 22 891 discloses an extremely advantageous variant of a sending/receiving unit, via which, on the one hand, the membrane of a vibration detector is excited to oscillate, and via which, on the other hand, the oscillations of the membrane are registered and converted into electrical signals. In each case, two sending and receiving electrodes are provided, which are, essentially, 90°-circular segments and are arranged on the same side of a disk-shaped, piezoelectric element. The piezoelectric element itself is homogeneously polarized and has a circular cross section. An inverter is provided for driving the piezoelectric element.